goalc/repl: cleanup of goalc/REPL code and some QoL improvements (#2104)

- lets you split up your `startup.gc` file into two sections
  - one that runs on initial startup / reloads
  - the other that runs when you listen to a target
- allows for customization of the keybinds added a month or so ago
- removes a useless flag (--startup-cmd) and marks others for
deprecation.
- added another help prompt that lists all the keybinds and what they do

Co-authored-by: water <awaterford111445@gmail.com>

common/util/diff.cpp

@@ -296,6 +296,8 @@ std::vector<std::string> SplitString(const ::std::string& str, char delimiter =
 }
 
 }  // namespace
+
+namespace google_diff {
 std::string diff_strings(const std::string& lhs, const std::string& rhs) {
   if (!lhs.empty() && !rhs.empty()) {
     const std::vector<std::string> lhs_lines = SplitString(lhs);
@@ -310,3 +312,4 @@ std::string diff_strings(const std::string& lhs, const std::string& rhs) {
 std::vector<std::string> split_string(const ::std::string& str, char delimiter) {
   return SplitString(str, delimiter);
 }
+}  // namespace google_diff